<!DOCTYPE html>
<html lang="en">
<head>
    <style>
        .cd{
            width: 100%;
            height: 500px;
        }

    </style>
    <meta charset="UTF-8">
    <title>Title</title>
    <link href="../css/all.css" type="text/css" rel="stylesheet">
</head>
<body>
<div class="cd">
<div class="background_red yd" style="width: 20%; height: 200px" id="4" >

</div>
<script>
    var id = document.getElementById("4");

</script>

</div>
</body>
</html>